The control of insulin pump therapy benefits greatly from knowing certain clinical variables including the insulin sensitivity factor and the carbohydrate factor, also known as carb factor or the insulin to carbohydrate ratio. Typically, these factors are determined by a manual method of administering insulin or carbohydrates and observing the effect of this administration on blood glucose level.
Calculation of insulin sensitivity factor is based on all of the units of insulin that a person takes in one day. Insulin sensitivity factor is also sometimes referred to as correction factor or correction bolus and is based on the drop in blood glucose level caused by one unit of insulin in units of milligrams per deciliter (mg/dL). Patients who are using insulin find that there are times when they need to make insulin adjustments in order to maintain blood glucose within target levels. In some cases, patients need to add more insulin at meal times to correct for high blood glucose. At other times, it may be necessary to correct a high blood glucose that is not associated with a meal. To utilize the insulin sensitivity factor to apply a corrective dose of insulin, it is necessary to know how many milligrams per deciliter one unit of insulin lowers the blood glucose. This value may vary with the individual patient and may also vary throughout the day or during times of illness. Generally, the goal is to apply a correction bolus that returns the blood glucose level to within thirty milligrams per deciliter of the target blood glucose level within three hours after the dose is given.
One method of calculating the insulin sensitivity factor is to take a three-day average of the total amount of insulin taken per day. This may be done by adding the basal daily total units of insulin taken in a given day to the bolus daily total units of insulin taken in that day to arrive at a total insulin value for that day. The insulin sensitivity factor is then determined by dividing a constant by the total daily insulin intake. Depending upon the type of insulin used, the constant varies. For some types of insulin the constant is considered to be 1,800; for other types of insulin the constant is considered to be 1,700; for yet other types of insulin, the constant is considered to be 1,500. In general, the 1500 sensitivity constant, sometimes referred to as the “1500 rule”, is used to estimate the blood glucose level drop, in milligrams per deciliter, for every unit of regular insulin taken. The 1800 sensitivity constant, sometimes referred to as the “1800 rule”, is used to estimate the blood glucose level drop, in milligrams per deciliter, for every unit of rapid-acting insulin taken. For example, if a patient has utilized thirty units total insulin daily and a correction constant of 1500 is used, 1500 divided by thirty equals fifty. This means that one unit of insulin would typically lower blood glucose for that patient by approximately 50 milligrams per deciliter (mg/dL).
The insulin sensitivity factor or correction factor is then used to calculate an insulin correction bolus dose. The correction bolus dose is calculated by subtracting from the current blood glucose level the target blood glucose and then dividing that difference by the insulin sensitivity factor. For example, if a patient has a current blood glucose of 200 milligrams per deciliter, and a target blood glucose of 100 milligrams per deciliter, 200 less 100 equals 100. 100 divided by the correction factor of 50 indicates that 2.0 units of insulin should be given for a correction dose.
The carbohydrate factor, also known as insulin to carbohydrate ratio or insulin to carb ratio, helps determine how much insulin should be taken to provide for proper metabolism of carbohydrates that would be eaten at a meal or in a snack. Carb ratios are calculated on a variable basis. For example, some patients might take 1.5 units of insulin for every carbohydrate choice, while others might take 1 unit of insulin for every 10 grams of carbohydrate that is expected to be eaten. Insulin-to-carb ratios vary from person to person and insulin to-carb-ratio may change over the course of treatment for some patients. Insulin to carb ratio may even vary depending upon the time of day.
Carb factors are commonly calculated using the “500 rule” (which is also sometimes known as the “450 rule” when using regular, non-fast acting insulin). Once the carb factor is known, the number of grams of carbohydrates in food that is to be eaten can be divided by the carb factor to determine how many units of bolus insulin is needed to cover metabolism of the carbohydrates that are to be eaten. This option provides patients flexibility in their food choices because the number of carbohydrates being ingested can be compensated for with a matching dose of insulin. According to the 500 or the 450 rule, an estimate of the number of grams of carbohydrates metabolized per unit of fast-acting insulin is determined. A constant of 450 is used for calculation with regular insulin. For example, when utilizing rapid-acting insulin, the constant of 500 is divided by the total daily dose of insulin to determine the grams of carbohydrates that are covered by one unit of rapid-acting insulin. The total daily insulin, sometimes abbreviated TDD, includes all fast-acting insulin taken before meals plus all long acting insulin used in a day. Correction doses of rapid-acting insulin taken to correct high blood glucose readings during the day should also be factored into the daily dosage.
The 500 rule is most accurate for those whose bodies make no insulin of their own and who receive 50 to 60 percent of their total daily dosage as basal insulin. For patients utilizing an insulin pump, the determined values used are then manually entered into the insulin pump where they are used to control insulin dosage. Examples of such pumps and various features that can be associated with such pumps include those disclosed in U.S. patent application Ser. No. 13/557,163, U.S. patent application Ser. No. 12/714,299, U.S. patent application Ser. No. 12/538,018, U.S. Provisional Patent Application No. 61/655,883, U.S. Provisional Patent Application No. 61/656,967 and U.S. Pat. No. 8,287,495, each of which is incorporated herein by reference.
However, none of the above estimation techniques for insulin sensitivity factor or carbohydrate factor is as accurate as would be ideal. Further, many patients are well-known to be non-compliant with medication regimens, particularly when the regimen becomes more complex or burdensome. Accordingly, there is still room for improvement in these areas.